This invention relates to the field of electrical supply systems as for vehicles and, more particularly, to systems for providing multiple output voltages with sufficient regulation on each output.
In many vehicles, particularly in trucks and heavy duty equipment, two different DC voltages are required with separately changing load levels. For example, a starter or cranking motor for such a vehicle may require a current of several hundred amperes at 24 volts for a brief period, then the 24 volt load drops sharply. Meanwhile, the alternator and a 12 volt battery will supply power for the accessories, and the ignition if so required.
Prior art solutions have included using two batteries with a series-parallel switch, creating a series connection for starting and a parallel connection for normal running operation. However, such a switch must be capable of carrying the heavy starting current, and the parallel connection is likely to have circulating currents which may cause serious problems. Another solution includes putting a tapped transformer between the alternator and the loads, but such a transformer will of necessity be heavy, bulky and expensive, not easily contained within the alternator housing. Another solution uses multiple isolated windings on an alternator with a relay having two pairs of contacts, one pair normally open and one pair normally closed. The relay coil is activated by an imbalance in the loads and switches the current to charge the batteries in series. However, none of these solutions has proven completely satisfactory, each having a problem with expensive and cumbersome structures and/or faulty regulation under widely varying load conditions.